In a variety of optical systems, it is necessary to assure that two or more separate optical signals are in phase. More particularly, it is necessary that their optical path difference (OPD) be made equal to within a fraction of a wavelength to assure proper coherence. The need for such coherence control is found in phased array laser transmitters/receivers and certain advanced array laser transmitters/receivers and certain advanced telescopes. For instance to realize the full potential of a phased array laser transmitter, the wavefronts emerging from its different telescopes should all lie on a common sphere, centered at a target. Thus, the individual laser beams need to be focused to the same range and pointed to a common point. The optical path lengths through the separate telescopes must be controlled so that the beams will add coherently at the receiver or the target. While means for controlling the OPD of a coherent beam are known in the art, a cost effective method for providing signals to operate the phase control apparatus has not been so clearly evident.
The phase control mechanism may be a mirror mounted on a piezo-electric or other electro-active substrate which can be moved in accordance with an applied signal voltage. One system of deriving a control signal for such a mirror actuator is described by R. R. Butts et al in "A Concept For A Phased Array Laser Transmitter ", Proceedings of SPIE, Volume 440, pgs. 188-125. Butts et al describe a system wherein samples from two beams under consideration are projected on a segmented detector array, the beams interacting to create an interference pattern. This interference pattern is the Fraunhofer diffraction pattern of the two beam samples (i.e. "double slit"). It comprises a series of bright and dark fringes modulated by a low frequency envelope. The detector array comprises a substantial number of individual photodetector segments, or pixels, which respond to the light and dark fringes by providing an electrical output signal proportional to the amount of light incident upon each pixel. Those signals are detected and the system then searches of the maximum intensity of the pattern. The maximum is used as the center of a subsequent pattern search: the date from the pattern search is processed; and a control signal is generated to modify the OPD such that the power levels are symmetrical about this central maximum. When this is achieved, the two optical beams are said to be in phase.
In a paper entitled "Analysis of Phase Measurement Algorithms Utilizing Two Beam Interference", Butts, Proceedings of SPIE, Volume 440, pgs. 130-134, a number of algorithms are described which are used in the analysis of OPD induced interference patterns.
The implementation of the Butts algorithm can be difficult. Butts' implementation employs many individual detectors, each of whose outputs must be sampled and all of whose outputs must be analyzed to determine where the maximum radiance exists. Sampling requires a clock cycle per detector, so the time required for the analysis is substantial, given the need for a large number of detectors to accurately detect the light and dark areas of the fringe pattern. Once each of these outputs is known, the system then requires considerable computing power to locate the region of interest in the interference pattern and perform the algorithm.
Harrell et al. in U.S. Pat. No. 4,942,581 entitled Optical Phase Detection and Control System, issued Jul. 17, 1990 discloses a system for determining the phase relationship between two coherent beams. Harrell's system combines beams to create an interference fringe pattern. Although the system appears to perform its intended function, it requires a complex optical system including costly mirrors requiring precise positioning.
Sum-frequency (SF) generation in nonlinear waveguides can be employed to convert infrared light to visible light, as well as to carry out signal processing functions such as wavelength separation and autocorrelation. Until recently, the SF power emitted from the surface of such nonlinear waveguides has been too low for practical application. However, a new multilayer waveguide structure, developed by Normandin et al. Electron. Lett. 26, 2089 (1990) has a SF generation efficiency several orders of magnitude larger than conventional homogeneous waveguides. As a result, there is a renewed interest in the application of SF generation in waveguides for various optoelectronic devices including spectrometers, correlators, and coherent visible light sources. In particular, the emission direction of the SF light relative to the surface normal varies with the difference in frequency of the two counter-propagating pump beams. High resolution spectrometers and wavelength division demultiplexers using nonlinear waveguides take advantage of this phenomenon. However, attention has not been given to the dynamics of the near field SF radiation pattern as the phase and frequency of the input beams is varied.
It is an object of the invention to provide a simplified optical phase detection system.
It is a further object of the invention to provide a simple method for detecting a change in the phase relationship between a plurality of signals.
In accordance with the invention there is provided, a method of detecting a change in the phase relationship between a plurality of signals comprising the steps of mixing the plurality signals in a nonlinear waveguide to produce a near field output pattern at the surface of the waveguide which corresponds to a phase relationship between the input signals; and, monitoring the near field output pattern at the surface of the waveguide for changes in the pattern, wherein a change in the pattern corresponds to a change in the phase relationship between the input signals.
In accordance with another aspect of the invention, there is provided a system for determining a phase relationship between a plurality of coherent beams comprising: non-linear waveguide means for providing a near field output pattern at the surface of the non-linear waveguide means in response to mixing four orthogonal polarized light beams within the waveguide, wherein two of the beams are counter propagating with two of the other beams; and detection means positioned to receive at least a portion of the near field output pattern and for detecting the intensity of the received portion of the near field output pattern at the surface of the waveguide.